1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of driving a liquid crystal display device.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are driven based on optical anisotropy and polarization characteristics of a liquid crystal material. Liquid crystal molecules have a long and thin shape, and the liquid crystal molecules are regularly arranged along in an alignment direction. Light passes through the LCD device along the long and thin shape of the liquid crystal molecules. The alignment of the liquid crystal molecules depends on the intensity or the direction of an electric field applied to the liquid crystal molecules. By controlling the intensity or the direction of the electric field, the alignment of the liquid crystal molecules is controlled to display images.
A related art LCD device and a driving method of the same will be described with reference to the accompanying drawings.
FIG. 1 is an equivalent circuit diagram of a related art LCD device.
In FIG. 1, the related art LCD device includes gate lines G1 to Gn, data lines D1 to Dn, switching elements T, liquid crystal capacitors CLC and storage capacitors Cst. The gate lines G1 to Gn and the data lines D1 to Dn cross each other to define pixel regions P. The switching element T, the liquid crystal capacitor CLC and the storage capacitor Cst are disposed at each pixel region P. A capacitance of the liquid crystal capacitor CLC is defined by a potential difference between a pixel voltage and a common voltage applied to liquid crystal.
In the LCD device of FIG. 1, scanning signals are sequentially applied to the gate lines G1 to Gn with time intervals, and the switching elements T connected thereto turn on. According to this, data signals from the data lines D1 to Dn are input to pixels through the switching elements.
More particularly, the scanning signals are sequentially applied to a first gate line G1 to an nth gate line Gn. When the scanning signal is applied to the first gate line G1, switching elements T, gate electrodes of which are connected thereto, turn on. At this time, selected data signals flow through the data lines D1 to Dn, and selected pixels become on states.
Here, the scanning signals are applied for a short time. To maintain charged amounts of the liquid crystal capacitors CLC until next scanning signals are applied, capacitances of the storage capacitors Cst are used.
If voltages having the same polarities are continuously applied to liquid crystal capacitors CLC, the liquid crystal of the liquid crystal capacitors CLC may be degraded to cause flickering or dimming of an image. According, to prevent the degradation of the liquid crystal and improve qualities of the image, the LCD device is driven by inversion driving methods, in which polarities of the liquid crystal capacitors CLC are regularly inversed.
The inversion driving methods include a frame inversion driving method, in which the polarities of the liquid crystal capacitors CLC are inversed every frame, a column inversion driving method, in which the polarities of the liquid crystal capacitors CLC are inversed every vertical line, a line inversion driving method, in which the polarities of the liquid crystal capacitors CLC are inversed every horizontal line, a dot inversion driving method, in which the polarities of the liquid crystal capacitors CLC are inversed every pixel region P, and so on.
FIG. 2 is a view of illustrating signals for explaining operation of an LCD device of FIG. 1 and shows a pixel voltage Vp and a common voltage Vcom. The LCD device may be driven by a dot inversion driving method.
In FIG. 2, the pixel voltage Vp and the common voltage Vcom are applied to the liquid crystal capacitor CLC of FIG. 1. The common voltage Vcom is a direct current (DC) voltage. The pixel voltage Vp is an alternating current (AC) voltage having positive and negative polarities alternately with respect to the common voltage Vcom.
In the dot inversion driving method, voltages having opposite polarities are applied to respective pixels adjacent to each other along horizontal and vertical directions. Further, the polarities are changed every frame. Accordingly, flickers are offset in the pixels adjacent to each other along the horizontal and vertical directions, the degradation of the liquid crystal can be prevented.
A structure of an array substrate for an LCD device according to the related art will be described hereinafter with reference to accompanying FIG. 3.
FIG. 3 is a cross-sectional view of schematically illustrating an array substrate for a twisted nematic (TN) LCD device according to the related art, which is driven with a normally white mode.
As shown in FIG. 3, the LCD device according to the related art includes a lower substrate 22 and an upper substrate 50, with a liquid crystal layer 14 is interposed between the lower substrate 22 and the upper substrate 50. Thin film transistors T, pixel electrodes 46, gate lines 13 and data lines 42 are formed on the lower substrate 22. A black matrix 52, red, green and blue color filters 54a, 54b and 54c and a common electrode 56 are formed on the upper substrate 50. The lower substrate 22 including the thin film transistors T, the pixel electrodes 46, the gate lines 13 and the data lines 42 may be referred to as an array substrate. The upper substrate 50 including the black matrix 52, the color filters 54a, 54b and 54c, and the common electrode 56 may be referred to as a color filter substrate.
The gate lines 13 and the data lines 42 cross each other to define pixel regions P. The thin film transistors T are disposed near respective crossings of the gate and data lines 13 and 42 and are arranged in a matrix.
Each pixel electrode 46 is disposed at each pixel region P and is formed of a transparent conductive material such as indium tin oxide (ITO) that has relatively high transmittance of light. The pixel electrodes 46 are connected to the thin film transistors T, respectively. The pixel electrodes 46 are also arranged in a matrix.
Each thin film transistor T includes a gate electrode 30, an active layer 34, and source and drain electrodes 36 and 38. The gate electrode 30 is connected to the gate line 13 and is supplied with pulse signals from the gate line 13. The source electrode 36 is connected to the data line 42 and is supplied with data signals from the data line 42. The data signals are provided to the pixel electrode 46 through the drain electrode 38 that is spaced apart from the source electrode 36 and that is connected to the pixel electrode 46. The active layer 34 is disposed between the gate electrode 30 and the source and drain electrodes 36 and 38.
In a TN LCD device, when voltages are not applied, liquid crystal molecules of the liquid crystal layer 14 are initially twisted with 90 degrees.
That is, the liquid crystal molecules adjacent to the upper substrate 50 have an angle of 90 degrees with respect to the liquid crystal molecules adjacent to the lower substrate 22, and the liquid crystal molecules therebetween are arranged with gradually decreasing changed.
First and second polarizers 62 and 64 are disposed at outer surfaces of the upper substrate 50 and the lower substrate 20, respectively. The first polarizer 62 has a light transmission axis perpendicular to a light transmission axis of the second polarizer 64. The light transmission axes of the first and second polarizers 62 and 64 are parallel to the liquid crystal molecules adjacent to the upper substrate 50 and the lower substrate 20, respectively.
In an off state when voltages are not applied, light from a backlight (not shown) passes through the second polarizer 64 and becomes linearly polarized light. The linearly polarized light is twisted with 90 degrees while passing through the liquid crystal layer 14 and transmits the first polarizer 62 to display white.
On the other hand, in an on state when voltages are applied, the liquid crystal molecules of the liquid crystal layer 14 are arranged perpendicularly to the upper and lower substrates 50 and 22.
Accordingly, light from the backlight passes the second polarizer 64 and the liquid crystal layer 14, but the light is blocked or absorbed by the first polarizer 62, the light transmission axis of which is perpendicular to that of the second polarizer 64, to thereby display black.
Meanwhile, in the LCD device of FIG. 3, an end portion of the pixel electrode 46 extends over the gate line 13, which is previously disposed, and the storage capacitor Cst includes the gate line 13 as a first electrode and the pixel electrode 46 overlapping the gate line 13 as a second electrode. At this time, it is importance to make the storage capacitor Cst have a enough capacitance.
However, in the LCD device, since the gate line 13 is used an electrode of the storage capacitor Cst, there may be signal delay of the gate line 13, and this lowers operation of the LCD device.
To solve the problem, another structure of an array substrate for an LCD device has been proposed, which further includes a storage line as the first electrode of the storage capacitor.
FIG. 4 is a plan view of an array substrate for an LCD device according to the related art.
In FIG. 4, gate lines 74 are formed on a substrate 70 along a first direction, and data lines 86 are formed along a second direction. The gate lines 74 and the data lines 86 cross each other to define pixel regions P.
A thin film transistor T is formed near by each crossing point of the gate and data lines 74 and 86. The thin film transistor T includes a gate electrode 72, an active layer 80, a source electrode 82 and a drain electrode 84. The gate electrode 72 is connected to the gate line 74 and receives scanning signals from the gate line 74. The active layer 80 is formed over the gate electrode 72. The source electrode 82 is connected to the data line 86 and receives image signals from the data line 86. The drain electrode 84 is spaced apart from the source electrode 82.
A common line is further formed. The common line includes a first portion 76a, a second portion 76b, a third portion 76c, a fourth portion 76d, and a fifth portion 76e corresponding to each pixel region P. The first portion 76a and the second portion 76b are parallel to the data line 86 and positioned at both sides of the data line 86, respectively, such that the data line 86 is disposed between the first and second portions 76a and 76b. The third portion 76c and the fourth portion 76d are parallel to the gate line 74 and cross the data line 86 in upper and lower areas of the pixel region P, respectively. The third and fourth portions 76c and 76d connect the first portion 76a and the second portion 76b. The fifth portion 76e connects the second portion 76b and another first portion 76a, i.e., a first portion of a next pixel region, across the pixel region P. The fifth portion 76e may be disposed near by the thin film transistor T. Therefore, the first portion 76a, the second portion 76b and the fifth portion 76e have one-united shape at each pixel region P.
A pixel electrode 88 is formed at each pixel region P and is connected to the drain electrode 84. The pixel electrode 88 overlaps the fifth portion 76e of the common line. The overlapped fifth portion 76e functions as a first electrode and the overlapped pixel electrode 88 functions as a second electrode to thereby form a storage capacitor. The pixel electrode 88 may partially overlap the first and second portions 76a and 76b. 
FIG. 5 is a view of illustrating signals for explaining operation of an LCD device of FIG. 4 and shows a pixel voltage Vp and a common voltage Vcom.
In FIG. 5, the pixel voltage Vp is applied to the pixel electrode 88, and the common voltage Vcom is applied to a common electrode (not shown), which is formed on a substrate opposite to the array substrate of FIG. 4. A storage capacitor voltage Vstg, which is applied to the common line 76a, 76b, 76c, 76d and 76e of FIG. 4, has the same value as the common voltage Vcom.
The thin film transistor T of FIG. 4 turns on by a scanning signal applied to the gate electrode 72 of FIG. 4, and the pixel voltage Vp is applied to the pixel electrode 88 of FIG. 4 through the thin film transistor T from the data line 86 of FIG. 4. The pixel voltage Vp alternates with respect to the common voltage Vcom.
By the way, in manufacturing the LCD device, there may be problems that the common line 76a, 76b, 76c, 76d and 76e and the pixel electrode 88 may short-circuit and particles may exist on a surface of a channel of the thin film transistor T. When a normally white mode LCD device displays black, pixels having the problems are shown white. Accordingly, these problems cause bright defects on a black image.
More detail explanation will be followed with reference to accompanying FIG. 6.
FIG. 6 is a cross-sectional view of an LCD device according to the related art and corresponds to the line VI-VI of FIG. 4.
In FIG. 6, the LCD device according to the related art includes a lower substrate 70 and an upper substrate 90, with a liquid crystal layer 98 is interposed between the lower substrate 70 and the upper substrate 90. Thin film transistors (not shown), pixel electrodes 88, gate lines (not shown), and data lines 86 are formed on the lower substrate 70. A black matrix 92, red, green and blue color filters 94a, 94b and 94c and a common electrode 96 are formed on the upper substrate 90.
As stated before, a common line is further formed on the lower substrate 70. The common line includes a first portion 76a, a second portion 76b, a third portion 76c of FIG. 4, a fourth portion 76d of FIG. 4, and a fifth portion 76e of FIG. 4 corresponding to each pixel region P. The pixel electrode 88 overlaps the fifth portion 76e of FIG. 4 to form a storage capacitor. The pixel electrode 88 also overlaps the first and second portions 76a and 76b. 
By the way, during a fabrication process, the pixel electrode 88 may short-circuit with the second portion 76b of the common line as shown in an area F of FIG. 6. Although shown in the figure, the pixel electrode 88 may short-circuit with the first portion 76a of the common line.
At this time, since the pixel electrode 88 is influenced by a storage capacitor voltage of the common line, the same voltage as the common electrode 96 is applied to the pixel electrode 88 to thereby transmit light. Accordingly, there exist bright defects on a black image when voltages are applied.
In addition, although not shown in the figure, there may be particles on a surface of a channel of the thin film transistor. At this time, the thin film transistor including particles should be separated, and the pixel corresponding to the thin film transistor results in a bright defect on the black image.
Recently, zero defects have been highly required, and it is essential to zero bright defects in the LCD device.
By the way, as mentioned above, since the TN LCD device is driven with the normally white mode, it is difficult to minimize the bright defects. Furthermore, low cell gap has been demanded due to needs of fast response, and the short circuit between electrodes causes loss of productivity.